Heterodyne optical network analysis is becoming an important tool for determining optical properties of optical devices, such as fiber Bragg gratings and optical fibers. Optical properties determined using a heterodyne optical network analyzer may include reflectivity, transmissivity, group delay, differential group delay and polarization dependent loss. A heterodyne optical network analyzer determines the optical properties of a device-under-test (DUT) from the amplitude and phase of interference signals. The interference signals are obtained by detecting the interference of two combined lightwaves. Typically, one of these lightwaves has been reflected off or transmitted through the DUT, and the other lightwave is a time-delayed version of the lightwave that was incident on the DUT.
A simple heterodyne optical network analyzer in a Mach-Zehnder interferometric configuration includes a tunable laser, an optical splitter, a DUT, an optical coupler, a detector, and an evaluation device. The tunable laser, which can be continuously tuned across an optical frequency range, generates an input lightwave having an optical frequency that sweeps over a predefined frequency range free of longitudinal mode hops. Longitudinal mode hops are laser frequency hops that can cause an abrupt change in the phase of an interferometric detection waveform, causing loss of DUT phase response information. The input lightwave is transmitted to the optical splitter, where it is split into two lightwaves that propagate along different optical paths. The lightwave following the first optical path travels through the DUT to the optical coupler, whereas the lightwave following the second optical path travels directly to the optical coupler. However, the second optical path has a different length than the first optical path. Thus, the lightwave on the second optical path experiences a positive or negative time delay relative to the lightwave on the first optical path. At the optical coupler, the lightwaves from the first and second optical paths are combined. The combined lightwaves are transmitted to the detector, where they interfere. To satisfy the Nyquist limit, the intensity of the interference signal is measured with a sampling rate at least twice the frequency of the interference signal. The measured intensity of the interference signal is then analyzed by the evaluation device to determine one or more optical properties of the DUT. As an example, the transmissivity of the DUT as a function of wavelength can be determined from the amplitude of the interference signal, which is proportional to the amplitude of the lightwave on the first optical path that traveled through the DUT. As another example, the group delay of the DUT can be determined by differentiating the phase of the interference signal with respect to frequency.
A concern with the described conventional heterodyne optical network analyzer is that under certain situations intensity noise may be incident on the optical detector along with the desired interference signal. The intensity noise can significantly degrade the measurements being made by the analyzer to determine the desired optical properties of a DUT. Often, intensity noise is quantified as relative intensity noise, or RIN. RIN is defined herein as the power spectral density of intensity or photocurrent fluctuations integrated over a predefined electronic bandwidth divided by the average optical power or photocurrent squared. It is understood that reducing intensity noise or detected intensity noise is equivalent to reducing RIN since both depend on the intensity noise. A typical laser used in a heterodyne optical network analyzer will have fluctuations in its output intensity due to a variety of reasons such as the well-known signal-spontaneous beat noise and multi-path interference (MPI). See Derickson, Fiber Optic Test and Measurement, Chapter 5 and Chapter 13, Prentice Hall (1998). These fluctuations can have a very broad spectral content, which can interfere with the measurements being made by the heterodyne optical network analyzer. Additionally, the DUT itself can generate intensity noise, which can mask or obscure the desired interferometric measurements.
In view of this concern, what is needed is a heterodyne optical network analyzer and method for reducing the effect of relative intensity noise in interferometric optical measurements for device characterization.